The last picture shows an old phone with "pt" in its display, which you get by typing 78 on the numeric keys.

The key is 736978,which must also be treated as pairs of hex digits. Now the message can be decoded by xor-ing the note with the key, starting over when the key has run out. Treat the resulting numbers as ASCII and the message reads:

The money is hidden in Xrspi Nrhqv. You are the key. Always Remember, it's not who you are underneath, but what you do that defines you.

The next step ...

... is to find out what "Xrspi Nrhqv" means. "You are the key" is important, of course, and it looks as if the place were encoded with a Vigenère cipher where the key is the name of the heir, which explains why that is also part of the question.

The note is probably meant to allow us to guess your identity, but I solved it by brute-forcing the code: I ran a dictionary against "Xrspi" and when the key "Bruce" produced "Wayne", I knew I was onto something: "Xrspi Nrhqv" decodes to "Wayne Manor" with the key "Bruce".

(The heir knows his name and can decode the message, but I'm not sure how we are supposed to find the key. The note says "It's not who you are underneath", but isn't Batman Bruce Wayne underneath?)

$\begingroup$Hm. But I didn't find the quote and I relied heavily on the work of others. (And I'm not such a big fan of "Do this and you'll get the tick".) Frankly, I don't quite see how the heir is supposed to identify himself by means of a quote related to him. If you want to wrap up your puzzle and are not satisfied with the solutions, you could write your own answer and accept it.$\endgroup$
– M OehmJan 7 at 18:06

$\begingroup$You got the right key, now decode the message! (For the last picture, you actually got the right number in a different way I intended for)$\endgroup$
– pirateJan 1 at 8:09

$\begingroup$For the benefit of other solvers, you should probably say that the ? ? is "Xrspi Nrhqv", which looks like another cipher. And it would be nice if you could explain how the cipher works.$\endgroup$
– M OehmJan 1 at 15:36

$\begingroup$I didn't want to give it all away before the mystery has been fully solved. Don't want to ruin the fun$\endgroup$
– j3AppJan 1 at 15:41

2

$\begingroup$Well, the moment you post a partial answer here, you basically say: That's how far I got, perhaps someone else has an idea about it. If you don't want to do share your findings, hold back the post until you've solved everything (and risk that someone else gets there in the meantime). At least that's how I see it.$\endgroup$
– M OehmJan 1 at 15:47